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The present invention relates to therapeutic radiation sources, and more particularly to miniaturized, optically driven therapeutic radiation sources.
In the field of medicine, therapeutic radiation such as x-ray radiation and xcex3-ray radiation is used for diagnostic, therapeutic and palliative treatment of patients. The conventional medical radiation sources used for these treatments include large, fixed position machines as well as small, transportable radiation generating probes. The current state-of-the-art treatment systems utilize computers to generate complex treatment plans.
Conventional radiation systems used for medical treatment utilize a high power remote radiation source, and direct a beam of radiation at a target area, such as a tumor inside the body of a patient. This type of treatment is referred to as teletherapy because the radiation source is located a predefined distance from the target. This treatment suffers from the disadvantage that tissue disposed between the radiation source and the target is exposed to radiation. Teletherapy radiation sources, which apply radiation to target regions internal to a patient from a source external to the target regions, often cause significant damage not only to the target region or tissue, but also to all surrounding tissue between the entry site, the target region, and the exit site.
Brachytherapy, on the other hand, is a form of treatment in which the source of radiation is located close to or in some cases within the area receiving treatment. Brachytherapy, a word derived from the ancient Greek word for close (xe2x80x9cbrachyxe2x80x9d), offers a significant advantage over teletherapy, because the radiation is applied primarily to treat only a predefined tissue volume, without significantly affecting the tissue adjacent to the treated volume. The term brachytherapy is commonly used to describe the use of radioactive xe2x80x9cseeds,xe2x80x9d i.e. encapsulated radioactive isotopes, which can be placed directly within or adjacent the target tissue to be treated. Handling and disposal of such radioisotopes, however, may impose considerable hazards to both the handling personnel and the environment.
The term xe2x80x9cx-ray brachytherapyxe2x80x9d is defined for purposes of this application as x-ray radiation treatment in which the x-ray source is located close to or within the area receiving treatment. An x-ray brachytherapy system, which utilizes a miniaturized low power radiation source that can be inserted into, and activated from within, a patient""s body, is disclosed in U.S. Pat. No. 5,153,900 issued to Nomikos et al., U.S. Pat. No. 5,369,679 to Sliski et al., and U.S. Pat. No. 5,422,926 to Smith et al., all owned by the assignee of the present application, all of which are hereby incorporated by reference.
The x-ray brachytherapy system disclosed in the above-referenced patents includes a miniaturized, insertable probe which is capable of generating x-ray radiation local to the target tissue, so that radiation need not pass through the patient""s skin, bone, or other tissue prior to reaching the target tissue. The insertable probe emits low power x-rays from a nominal xe2x80x9cpointxe2x80x9d source located within or adjacent to the desired region to be affected. In x-ray brachytherapy, therefore, x-rays can be applied to treat a predefined tissue volume without significantly affecting the tissue adjacent to the treated volume. Also, x-rays may be produced in predefined dose geometries disposed about a predetermined location. X-ray brachytherapy offers the advantages of brachytherapy, while avoiding the use and handling of radioisotopes. Also, x-ray brachytherapy allows the operator to control over time the dosage of the delivered x-ray radiation.
X-ray brachytherapy typically involves positioning the insertable probe into or adjacent to the tumor, or into the site where the tumor or a portion of the tumor was removed, to treat the tissue adjacent the site with a local boost of radiation. X-ray probes of the type generally disclosed in U.S. Pat. No. 5,153,900 include a housing, and a hollow, tubular probe or catheter extending from the housing along an axis and having an x-ray emitting target at its distal end. The probe may enclose an electron source, such as a thermionic cathode. In another form of an x-ray brachytherapy device, as disclosed in U.S. Pat. No. 5,428,658, an x-ray probe may include a flexible probe, such as a flexible fiber optic cable enclosed within a metallic sheath. In such a flexible probe, the electron source may be a photocathode. In a photocathode configuration, a photoemissive substance is irradiated by a LED or a laser source, causing the generation of free electrons. Typically, the flexible fiber optic cable couples light from a laser source or a LED to the photocathode.
The power requirements for the miniaturized x-ray brachytherapy systems can be reduced by optically driving the thermionic cathodes in the electron sources, instead of ohmically heating the thermionic cathodes. U.S. patent application Ser. No. 09/884,561 (identified by Attorney Docket Nos. PHLL-155 and hereby incorporated by reference)(hereinafter the xe2x80x9cPHLL-155xe2x80x9d application) discloses a miniaturized therapeutic radiation source that includes a reduced-power, increased efficiency electron source that is optically driven. The PHLL-155 application discloses an electron source that includes a thermionic cathode having an electron emissive surface. The PHLL-155 application discloses using laser energy to heat the electron emissive surface of the thermionic cathode, instead of heating the electron emissive surface of the thermionic emitter using conventional ohmic heating. In this way, electrons can be produced in a quantity sufficient to produce the electron current necessary for generating therapeutic radiation at the target, while significantly reducing the power requirements for the therapeutic devices. Electrons can be generated with minimal heat loss, without requiring a vacuum-fabricated photocathode.
Generally, when treating a targeted treatment region, such as a tumor or the body cavity or a tumor, it is desirable to uniformly radiate the entire surface of the tumor or the tissue lining the cavity, in such a way that an isodose contour is coincident with the surface of the body cavity. An isodose contour is a surface in which the absorbed radiation energy is equal at every point on the surface.
Even though the x-ray brachytherapy systems described above can generate x-rays local to the target tissue, it is difficult to provide a uniform, or other desired, dose of radiation to an irregularly shaped target tissue, using these systems. These x-ray sources generally act as point sources of therapeutic radiation. The intensity of the radiation from a point source decreases uniformly with approximately the square of the distance (R) from the source (i.e., 1/R2). Since body cavities, or the beds of resected tumors, are not generally spherically symmetrical, a point source within a body cavity or central to the resected tumor bed will not deliver a uniform dose of radiation to the tissue lining of the cavity or the bed. Similarly, for a non-spherical tumor, a point source at the tumor center will not deliver radiation with an isodose contour matching the peripheral surface of the tumor.
It is desirable to provide a uniform dose of radiation to the isodose contours that match the peripheral surface of a desired treatment region, using an optically driven, high efficiency x-ray brachytherapy system. It is also desirable to increase the efficiency of the miniaturized, optically driven radiation sources, described above.
It is an object of this invention to increase the uniformity and intensity of the x-ray radiation delivered by an optically driven x-ray brachytherapy system. In particular, it is an object of this invention to increase the uniformity and intensity of the x-ray radiation from an optically driven x-ray brachytherapy system, by altering the geometry and configuration of the x-ray target.
The present invention is directed to an optically driven x-ray source having a laser-heated thermionic cathode. In the present invention, the geometry and configuration of the x-ray target are designed to substantially increase the uniformity of the x-ray radiation delivered by the x-ray source onto a treatment region. In particular, an x-ray emissive target having a non-planar target geometry is utilized, in order to deliver a more uniform and more intense flux of x-ray radiation to a treatment region.
The present invention features a therapeutic radiation source that includes a radiation generator assembly, a source of optical radiation, and an optical delivery structure such as a fiber optic cable. The radiation generator assembly includes an electron source for emitting electrons to generate an electron beam along a beam path, and a target positioned in the beam path and adapted to emit therapeutic radiation in response to incident accelerated electrons from the electron beam. The electron source includes a thermionic cathode having an electron emissive surface. The fiber optic cable is adapted to transmit optical radiation, incident on an proximal end of the cable, to a distal end of the cable. The fiber optic cable directs a beam of the transmitted optical radiation upon the electron emissive surface of the cathode. The beam has a power level sufficient to heat at least a portion of the surface to an electron emitting temperature, so as to cause thermionic emission of the electrons from the surface.
The present invention features a non-planar configuration for the x-ray emitting target, in contrast to a planar, disk-shaped target. For example, the target may have a substantially conical shape, a substantially hemispherical shape, a substantially spherical shape, or a substantially convex shape. The target includes a thin film made of an x-ray emissive material, and an x-ray transmissive support structure. By fabricating the target in a non-flat configuration that includes an x-ray emissive film supported on an x-ray transmissive structure, the radiation pattern around the target can be rendered more uniform and more intense, as compared to x-ray targets in the prior art.